The invention is concerned especially with high performance diffraction grating-based optical spectrum analyzers (OSA) and monochromators. U.S. Pat. No. 5,233,405 (Wildnauer et al.), for example, discloses a double-pass, double-filtering monochromator using a Littrow configuration that has a waveplate positioned in the middle of the monochromator so that the light impinges upon the grating a second time with a polarization state orthogonal to the state of polarization when it impinged upon it the first time to reduce the PDL in combination with using a Littrow configuration with a grating having a small number of lines, which limits the resolution for a given size.
Optical spectrum analyzers that use different techniques for polarization management to achieve better resolution for a given size have been disclosed in U.S. Pat. No. 5,886,785 issued Mar. 1999 naming Lefevre et al. as inventors and in U.S. patent application Ser. No. 20010030745 filed Jan. 2000 naming He et al. as inventors.
Both their designs have advantages and weaknesses that are complementary, but they both suffer from important limitations with regard to the optical rejection ratio (ORR) and, at different levels, the noise floor. In the case of the He et al's OSA described in U.S. 20010030745, these limitations arise from the fact that the light is filtered only once, even after two passes on the grating.
In one embodiment (shown in their FIG. 8), Lefevre et al. use a second retro-reflector to double the number of passes of the grating. Although this leads to a reduced spectral linewidth response, the resulting optical spectrum analyzer still has a limited level optical noise floor and its optical rejection ratio (ORR) is not improved; it is, in fact, degraded by the extra loss incurred while the noise floor remains unchanged.
In U.S. Pat. No. 6,337,940, Lefevre discloses an OSA having the same basic features as that shown in FIG. 8 of U.S. Pat. No. 5,886,785, but which also filters the light a second time. This approach has the advantage of very high ORR filter response close to the peak, but the ultimate noise floor is limited by back-reflection from the common input and output lens, a limitation that becomes important when multiple signal wavelengths are to be analyzed, as is the case in DWDM systems that, in practice, require the high ORR. Other limitations include the use of large expensive components, the size/cost of the polarization beam splitter (120) within the monochromator section limiting the ultimately achievable filter response linewidth and the need for the components to be precisely polished to ensure that the optical beams of both polarizations can be recombined at the output of the monochromator.
While it might be possible to improve the spectral linewidth response for both these approaches by increasing the component dimensions (namely the lenses' focal lengths and the diffraction grating surface and, for Lefevre et al., the polarizing beam splitter), it would be at the expense of a larger occupied volume, higher cost and greater mounting difficulty.